This invention relates to a vacuum interrupter and, more particularly, to the assembly of shield assemblies of such an interrupter.
Vacuum interrupters comprise an evacuated envelope of substantially cylindrical configuration and opposing end portions. The end portions support, respectively, a movable contact rod and a fixed contact rod. The envelope comprises insulating members, for insulating the movable and fixed contact rods, and metallic members. During operation, i.e., separation and opening of the contacts, arcing occurs that produces metal vapors. Metallic shields are therefore arranged within the envelope to prevent the metal vapors from depositing on the insulating members. Such shields are usually arranged concentrically about the common center axis of the contact rods.
In some configurations, shields are secured to metallic cylindrical wall portions of the envelope. One such arrangement is disclosed in U.S. Pat. No. 4,081,640-Rich, which is assigned to the assignee of the subject application. This illustrates metal shields secured, i.e., welded, to a metallic cylindrical wall member. Specifically, the shields have a cylindrical wall portion that is secured to the inner surface of the central cylindrical wall portion of the envelope.
Copending patent application Ser. No. (796,149)-Sofianek et al, also assigned to the same assignee, is related to the subject application and discloses a shield arrangement that incorporates the subject invention. The Sofianek application discloses an interrupter whose envelope comprises a central ceramic insulator of cylindrical configuration and metallic end members, i.e., end bells, that are substantially bell-shaped. The end bells have cylindrical wall portions. End shields also having a cylindrical wall portion are secured to the inner surface of the cylindrical wall portions of the end bells.
The shields must be properly positioned along the longitudinal axis of the interrupter. Thus, they must be accurately positioned and secured to the cylindrical wall portion of the envelope in a plane whose longitudinal position is accurately defined.
Certain methods and arrangements for positioning and securing the end shields have deficiencies. For example, the shields may be welded at prescribed positions prior to brazing of the interrupter. However, prepositioning the weld locations and welding are time consuming and costly. Further, welding the shields to the end bell generates heat that produces stresses in the end bell. These stresses could produce breaks in the seal between the end bell and the insulating casing. Alternatively, fixtures might be employed for positioning and brazing the shields. However, such fixtures can not be utilized in certain methods of interrupter assembly. Even when usable, they are undesirable. For example, in some cases, the fixtures themselves might be brazed to the interrupter components. This results in unnecessary rejects.